Conservationists work to expose underwater hull

The hull of submarine HL Hunley in Charleston, South Carolina, hasn't seen daylight in more than 150 years.

Now, conservationists based at Clemson University's Warren Lasch Conservation Center are trying to expose it -- by soaking it in a caustic 13.5ph sodium hydroxide bath. Sodium hydroxide -- which has never been used on such a large artefact before -- will dissolve the 25mm-thick layer of shell and sand that formed on the submarine's hull during the 136 years it spent on the seabed near Charleston Harbour, where it sank on the night of February 17, 1864. It will also help to remove a century's worth of sea salt from the iron hull. "If we expose the hull to air with the salt in there, it would start corroding, and in a few years we would be left with a big pile of dust," explains the lab's director Stephanie Crette. To prevent this happening, Hunley has been kept in fresh water for the past 14 years, while over 3,000 artefacts were removed. The bath will take about five years, after which the sub will be displayed outside for the first time since it sank.

This article was first published in the November 2014 issue of WIRED magazine